Duplex printing, printing on both sides of a substrate such as a sheet of paper, is usually performed using a multi-pass system. After a substrate has received one image during a first pass through an imaging station, the substrate is inverted and a second image is produced on the other side during a second pass. The 5090 duplicator and the DocuTech.RTM. Production Publisher, both of which are products of the Xerox.RTM. Corporation, are examples of duplex printing systems.
For various reasons the registration of images on opposites sides of a substrate is not always accurate. The result is image offsets between the images on the front and on the back of a substrate. To reduce these image offsets, active registration systems, systems that sense the substrate position and which correct that position as necessary, have been used. For example, U.S. Pat. No. 4,971,304 to Loftus discloses an apparatus for deskewing and side registering a copy sheet. The apparatus disclosed therein includes copy sheet drivers that are independently controllable to selectively provide differential and non-differential driving of the copy sheet in accordance with the copy sheet position as sensed by at least three sensors. In addition, Loftus discloses the use of a fourth sensor to measure the position of the sheet after deskew and side-registration with respect to the position of a latent image on a photoreceptor and with respect to a transfer station. Similar deskewing and side registration systems have been disclosed in U.S. Pat. Nos. 5,169,140; 5,156,391; 5,094,442; 5,078,384; 5,172,907; and 5,278,624. Other registration systems which are mechanical in nature deskew and side register by urging a copy sheet against a guide or gate. Examples of such mechanical registration systems are disclosed in U.S. Pat. Nos. 4,416,534; and 4,519,700.
While the multipass duplex registration systems used in the 5090 duplicator and the DocuTech.RTM. Production Publisher systems are generally successful they are not as precise as some end users might desire. One specific problem with image registration systems relates to registration errors that develop because of substrate dimension tolerances. For example, a given substrate might vary .+-.1 mm from a nominal dimension. Because most registration systems use the same registration edge regardless of the plex of a sheet (i.e. front or back), dimension variations translate into registration errors when duplex printing. Therefore a multi-pass duplex registration system that corrects for dimensional variations in a substrate's dimensions would be beneficial in improving image registration.